Efficient second harmonic generation (SHG) is desired for stabilizing octave-spanning frequency combs with the self-referencing technique. The low pump power level from a single (or a few) comb lines typically requires an amplifier before the SHG conversion to produce sufficient second-harmonic light for the self-referencing technique. Bulk SHG crystals are often used with free-space input and output beams. For example, many traditional systems use periodically-poled crystals for quasi phase-matching of the pump and signal light.
Recent developments have integrated similar SHG devices in waveguides, rather than free-space. Waveguides allow for a dramatic increase of the pump light intensity due to the smaller effective area of the optical mode, leading to a greater efficiency. These waveguide SHG devices generally are implemented with a straight waveguide section and facet couplers at the input and output. Some traditional systems have used a repetition of bends with alternating bend radius sign with the intent to broadening the conversion efficiency spectrum while decreasing the peak efficiency or used a bend meander to increase the interaction length, though the bends have the same waveguide width as the straight sections. Unfortunately, these traditional systems suffer from a phase-mismatch in the straight and bend sections and as a result, the conversion efficiency is very low.